Centrifugal pumps typically employ a wheel with vanes or blades called an impeller. The impeller is mounted on a shaft for rotation within a housing or casing. Fluid is supplied to the eye or center of the impeller through an inlet. As the impeller rotates at high speed, the fluid is impelled around the inner circumference of the casing, thereby applying centrifugal force to the fluid. The housing has a volute shape and is graduated in size from the impeller to an outlet, whereby the flow of the fluid is changed from a high-velocity, low-pressure stream at the inlet to a low-velocity, high-pressure stream at the outlet.
It is common practice to provide a stationary ring between the impeller and the casing. A radial clearance, usually about 0.005 inches to about 0.006 inches, exists between the casing ring and the impeller to permit internal recirculation of the fluid within the pump. A radial clearance, usually about 0.002 inches, exists between the casing ring and the casing. Because the radial clearance between the casing ring and the casing is so small, the casing ring cannot move in a radial direction relative to the casing. Thus, the casing ring, which is made from a softer material than the impeller, becomes worn as the impeller, due to dynamic forces that result in shaft deflection, moves off center and contacts the casing ring. As the radial clearance between the casing ring and the impeller increases due to the wearing of the casing ring, the internal recirculation within the pump increases, thereby adversely affecting pump performance and efficiency.